Screw press for drying elastomeric materials

ABSTRACT

IN THE WATER-VAPORIZING SECTION OF AN OPEN-BARREL SCREW PRESS DRYER, USEFUL FOR REMOVING WATER FROM ELASTOMERS, THE COMBINATION OF A SCREW POSITIONED COAXIALLY WITHIN A SLOTTED BARREL, THE SCREW AND THE BARREL BEING SO PROPORTIONED THAT THE FREE VOLUME BETWEEN THE SCREW AND BARREL IS FROM 8 TO 18 PERCENT OF THE TOTAL AVAILABLE BARREL VOLUME.

United States Patent [72] Inventor Stanley G.Smith Beaumont. Tex. [21] Appl. No. 759.727 [22] Filed Sept. 13. 1968 [45] Patented June 28. 1971 v [73] Assignee E. I. du Pont de Nemours and Company Wilmington. Del.

[54] SCREW PRESS FOR DRYING ELASTOMERIC MATERIALS 3 Claims, 1 Drawing Fig.

[52] 11.8. C1 [00/117, 100/127,100/145.18/12, 34/14 151] lnt.Cl 1330b 9/12 [50] Field of Search 100/117, 93.97.98, 145-150; 18/12 ($191.2 (FM). 125 l. 125 mi 34/12. 14

[56) References Cited I UNITED STATES PATENTS 2.149 73'6 3/1939 Hiller et a1. 100/150X Primary Examiner- Peter Feldman Anomey- Fred C. Carlson ABSTRACT: In the water-vaporizing section of an open-barrel screw press dryer, useful for removing water from elastomers, the combination of a screw positioned coaxially within a slotted barrel, the screw and the barrel being so proportioned that the free volume between the screw and barrel is from 8 to 18 percent of the total available barrel volume.

DEIATEIIIG SECTION SCREW PRESS FOR DRYING ELASTOMERIC MATERIALS BACKGROUND OF THE INVENTION This invention pertains to improvements in apparatus for dewatering and drying elastomeric crumb materials, More particularly it is concerned with improvements in apparatus of the type described in U.S. Pat. No. 3,225,453 issued to A. H. Burner. This patent describes a screw press dryer consisting of a dewatering section, wherein water is mechanically removed continuously from a mass of wet elastomer crumb, and a vaporizing section wherein the remaining water is evaporated off by heating the elastomer mass. This screw press dryer is especially adapted to the drying of natural rubber and synthetic rubber such as styrene-butadiene copolymers. With newer types of elastomers such as terpolymers of ethylene, propylene and hexadiene (EPDM), however, the drying action of screw-press dryers made in accordance with the teachings of the Burner patent and sold commercially by its assignee has been found to be capable of unexpected improvement. Such dryers have a proportioning of screw assembly and slotted barrel size in the vaporization section such that the free volume is above about 22 percent.

SUMMARY OF THE INVENTION Now according to the present invention it has been found that the foregoing and related problems of the prior art can be solved, improved drying of such elastomers as natural rubber and styrene-butadiene polymers can be attained, and complete dryingof EPDM-type elastomers can be achieved in a screw-press dryer of the type described in Burner US. Pat. No. 3,225,453 if the screw and barrel in the vaporizing section are so proportioned that the free volume between the screw and barrel is about from 8 to 18 percent of the total available barrel volume.

BRIEF DESCRIPTION OF THE DRAWING In the drawing, the FIG. is a longitudinal cross section of a screw press dryer of the invention in which:

A shaft 20, mounted at each end in bearings, not shown, carries a feed worm 50 adapted to pick up the elastomer crumb from a feed hopper, not shown here but illustrated as 12 in FIG. 3 of US. Pat. No. 3,225,453, and move it towards the right into the expression chamber or dewatering section where waterv is mechanically removed from the crumb. Mounted concentrically on shaft 20 to form a screw assembly are a'plurality'of collars 62 and worm members 65, the perimeters of the collars and worm members being shaped to match the contour of a cageor barrel-section within which the screw is to revolve.

The barrel section is'made up of a plurality of longitudinal screen bars of rectangular cross section spaced apart a few thousandths of an inch to form slots through which water which is squeezed out of the elastomer crumb can escape. The detail of the slotted-barrel construction is not shown in the drawing, but is known to the art and is illustrated in FIGS. 1 and 3 of Burner US. Pat. 3,276,354 wherein 55 represents the bars and 56 the spacers. Water escaping through the slotted barrel shell is drained away by suitable'means, not shown. In the barrel shell are mounted one or more bushings 100, the inner diameters of which are preferably a few thousandths of an inch greater than the worm flightswhich they surround. Normally this spacing remains as running clearance. If some force occurs causing the shaft to deflect, the worm flight affected can run in contact withthe annular inner surface of its related bushing, thereby minimizing shaft deflection and consequent wear'on the endbearings. I

Each barrelor cage section isprovided with a center barj 57 at its top and bottomwhich serves to retain the screen bars in the section. Certain of the center bars are provided with inward-extending lugs 60, sometimesreferredto as breaker lugs, opposite collars 62 on the shaft 20. These lugs are adapted to engage the clastorner being forced toward them by thc worm flights and to prevent rotation of the material along the extent of their length axially of the cage, causing the elastomeric material to be fed toward the discharge end. Some of the collars 62 are of uniform cross section along their lengths while others are formed to increase in cross section toward the discharge end of the cage, thus forming with the cage walls a passage of decreasing cross section through which the material is forced, resulting in mechanical compaction and compression of the material being dried. The compaction forces are increased and directed radially by collars 62. This substantial radial pressure has the effect of forcing any liquid water present through the drainage openings of the cage sections.

The worm members 65 are provided with notches or other discontinuities 67 in the worm flights. This notched worm construction enhances the mechanical working of the material being dried.

The portion of the slotted barrel or cage assembly nearest the feed end is referred to herein as the dewatering section while that closest to the discharge end is called the vaporizing section. In the latter, heat may be supplied, by means not shown, whereby water not removed in the dewatering section is vaporized and removed as steam. The frictional heat generated by mechanical working and tearing of the material being dried contributes significantly to the total heat available for this drying action.

The free space 1 between the screw assembly and the slotted barrel in the dewatering section substantially exceeds the free space 2 in the vaporizing section. According to this invention the latter is critical and is limited to from 8 to 18 percent of the total available space in that section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS wherein the water-vaporization occurs. This dual arrangement is illustrated in Example 3 hereinbelow.

The dryer is of the open-barrel typethat is, the barrel wall has longitudinal openings or slots thaerein, adapted to permit water or water vapor to escape but to retain substantially all of the polymer being dried. Such barrels are already well known in the art, and are sometimes referred to as slotted barrels.

The free volume of the water-vaporizing section of a slotted barrel screw press is calculated by subtracting the volume occupied by the screw from the total volume of the barrel, both volumes being in said water-vaporizing section only. According to the present invention it has been found that the free volume is critical and should be from 8 to 18 percent of the total volume of the barrel for adequate drying of EPDM polymers and polymers of similar rheological properties. This means that there is relatively close clearance between the screw and the barrel and as a consequence the polymer is very tightly squeezed and is subjected to frictional forces hitherto not attained in screw-press dryers of conventional design.

It is emphasized that the free volume limits apply to the water-vaporization section of the dryer-that is, the area in which there is substantially no further mechanical removal of liquid water. With more than 18 percent free volume the section does not drive off all of the water in an ordinary wet polymer of theEPDM-type. It will be understood that the presence of water'is incidental to the process of manufacturing the polymer wherein the polymer in the form of crumb is washed with water to remove impurities resulting from its manner of preparation. Ordinarily the crumb as fed to the screw press dryer will contain about from 40 to 60 percent of water and mostof it will be occluded so that it cannot simply be drained off. The mechanical action in the dewatering sectionreduces the water content to about from 5 to 12 percent in thepolymer as it goes to the water-vaporization section.

the same limitations as herein described but on the contrary is ordinarily substantially more than 18 percent.

The invention will be better understood by reference to the following illustrative examples.

EXAMPLE 1 This example describes a shaft configuration, employed on a single screw press mechanical dryer of the open barrel type for water removal from elastomers. which limits the free volume between the screw and the barrel in the water vaporizing section to 14 percent of the available free barrel volume. This screw press drys elastomer crumb containing approximately 50 percent water by expressing water from the polymer until it contains to 12 percent residual moisture. and then heating the elastomer above 150 C. to vaporize the remaining water. Squeezed water and steam are exhausted through the side of the barrel. The vaporizing section in this example consists of the last four sections (44 inches) of an eight section (88 inch) screw press, although it can utilize as few as three sections or as many as five sections.

In the vaporizing section this screw configuration is composed of 13-inch drum chamber plain and notched worms of conventional design l3'/s-inch flight diameters, and 12 and 13- inch diameter collars arranged in alternating fashion to restrict the free volume to 14 percent of the available volume in a 14-inch diameter barrel. Worm pitch of 3 inches is used in the vaporizing section, although minor variations (less than 1 inch in either direction) in this dimension function satisfactorily. Screw length devoted to collars in the alternate collarworm-collar stacking arrangement amounts to 42 percent of the total length in the vaporizing section. This invention functions satisfactorily with 35 percent to 50 percent of the shaft length devoted to collars. Breaker bars of conventional design are used with this configuration as described above with reference to the drawing. This shaft arrangement can be operated at speeds of 87 to 160 r.p.m. to permit optimum performance with a particular type of elastomer. Drying rates demonstrated in a single screw press equipped with this invention range from 1000 to 3000 lb./hr. of FPDM polymer having a Mooney viscosity of 45ML-4:250 F. and 1000 to 2200 lb./hr. of EPDM polymer having a Mooney viscosity of 30ML-4z250 F.

EXAMPLE 2 This example describes a shaft configuration, employed on a single screw press mechanical dryer of the open barrel type for water removal from elastomers, which limits the free volume between the screw and the barrel of the water vaporizing section to l 1 percent of the available free barrel volume. This screw press drys elastomer crumb containing approximately 50 percent water by expressing water from the polymer until it .contains 5 to percent residual moisture; and then heating the elastomer above 150 C. to vaporize the remaining water. Squeezed water and steam are exhausted through the side of the barrel. The vaporizing section in this example consists of the last four sections (44 inches) of an eight section (88 inch) screw press, although it can utilize as few as three sections or as many as five sections.

In the vaporizing section this screw configuration composed of l3-inch drum-diameter plain and notched worms of conventional design with l3 /a-inch flight diameters, 13-inch diameter collars, and 13-inch to l3 -inch diameter rising collars arranged to restrict the free volume to l 1 percent of the available volume in a 14-inch diameter barrel; ln this example the rising collars are used in the five collar positions immediately prior to press discharge to reduce the free volume and apply additional mechanical work to the elastomer. The number, position, and dimensions of these conventionally designed rising collars can be varied along as the free v alume is maintained between 8 and 18 percent of the available barrel volume. Worm pitch of 3 inches is used in the vaporizing section, although minor variations in this dimension function satisfactorily. Screw length devoted to collars amounts to 42 percent of the total length in the vaporizing section in this example. This invention functions satisfactorily with 35 percent to 50 percent of the shaft length devoted to collars. Breaker bars of conventional design are used with the configuration as shown in the drawing. This shaft arrangement can be operated at speeds of 87 to 160 r.p.m., the particular speed being chosen to permit optimum performance with a particular type of elastomer. Drying rates demonstrated on EPDM elastomers in a single screw press equipped with this invention include:

EXAMPLE 3 This example describes a shaft configuration, employed on the second of two screw presses of the open barrel type operating in series for water removal from elastomers, which limits the free volume between the screw and the barrel of the water vaporizing section to 18 percent of the available free barrel volume. Elastomer crumb containing approximately 50 percent water is dewatered in the first screw press to 5 to 20 percent residual moisture by squeezing the crumb between the shaft and the barrel. The second screw press receives the partially dewatered crumb and provides additional squeezing to remove all the expressible (to 5l0 percent residual) water. It then heats the elastomer above C. to vaporize the remaining water. Water is exhausted through the barrel of the first screw press and water and steam are exhausted through the barrel of the second screw press. In this example the vaporizing section of the second machine consists of the last five Sections (55 inches) of an eight section (88 inch) screw press. If more water is removed-in the first screw press then the vaporizing section of the second machine can include six, seven, or eight sections of an eight section screw press. Similarly if the second screw press only had six sections then the vaporizing section would consist of either three, four, five or six section.

In the vaporizing section of the second press this screw configuration is composed of 12- and l2 /-inch drum diameter plain and notched worms of conventional design with 13%- inch flight diameters, 12-inch diameter collars, 12-inch to 12 /-inch diameterrising collars, and lZ A-inch to l3 /4-inch diameter rising collars arranged to restrict the free volume to 18 percent of the available volume in a 14-inch diameter barrel.

In this example the rising collars are used in the collar positions immediately prior to the press discharge to reduce the free volume and apply additional mechanical work to the polymer. The number, position, and dimension of these rising collars can be varied as long as the free volume is maintained between 8 and 18 percent of the available barrel volume. Worm pitch of 3 inches is used exclusively in the vaporizing section, although minor variations in this dimension would function satisfactorily. Screw length devoted to collars amounts to 40 percent of the total length in the vaporizing section in the example. This invention functions satisfactorily with 35 percent to 50 percent of the shaft length devoted to collars. Breaker bars of conventional design are used with this configuration as'discussed above in reference to the drawing. This shaft arrangement can be operated at speeds of 87 to r.p.m., the particular speed being chosen to permit optimum performance with a particular type of elastomer. Drying rates demonstarated in a two-press dryingline with the shaft arrangement of this invention installed in the second press range form 3000 to 4600 lb./hr. of EPDM polymer having a Mooney viscosity of 70M L-4:250 F.

being so proportioned that the free volume between the screw and barrel is from 8 to l8 percent of the total available barrel volume.

2. A screw press dryer of claim 1 in which the diameter of the screw at the discharge end is larger than at the input end.

3. A screw press dryer of claim 1 in which the watervaporizing section is immediately preceded by a dewatering section having a greater proportion of free volume to total available volume than does said water-vaporizing section. 

